The present invention relates to a biaxially oriented polyester film and to a flexible disk comprising the same as a base film. More specifically, it relates to a biaxially oriented polyester film which has excellent handling properties and is free from a dimensional change when or after it is processed for use in a flexible disk and to a flexible disk which comprises the same as a base film and has excellent electromagnetic conversion characteristics.
A biaxially oriented polyester film typified by a biaxially oriented polyethylene terephthalate film and a biaxially oriented polyethylene naphthalene dicarboxylate film is widely used as a base film for magnetic recording media due to its excellent physical and chemical properties. In recent years, an adhesive polyester film prepared by forming an adhesive layer on a biaxially oriented polyester film to improve its adhesion to a magnetic layer and simplify the production process of a magnetic recording medium (improving production efficiency) has been becoming the main stream of the market.
Meanwhile, along with the popularization of personal computers and the like, floppy disk drive units and floppy disks have been widely used as storage devices and the capacity and density of floppy disks have been increasing. Therefore, a base film having a more flat surface and better handling properties is now desired.
Further, due to the recent improvement of the performance of notebook type personal computers, personal computers equipped with a floppy disk drive and the like involve such a problem as high temperature inside the drive. Therefore, dimensional stability is an extremely important factor for a high-density magnetic recording medium.
A base film having a more flat surface is now in demand and the further improvement of blocking resistance is desired. In addition, along with an increase in the flatness of the surface of the base film, coating defects readily exert an influence upon the electromagnetic conversion characteristics of a magnetic tape. Accordingly, an adhesive polyester film free from coating defects and having excellent coatability is desired from the market.
It is an object of the present invention to provide a biaxially oriented polyester film which is flat, excellent in handling properties and free from a dimensional change when and after it is processed for use in a flexible disk.
It is another object of the present invention to provide a biaxially oriented polyester film which is useful as a base film for a flexible disk because its dimensional stability is not deteriorated by tension applied when a magnetic layer is formed.
It is still another object of the present invention to provide a biaxially oriented polyester film which has excellent adhesion, blocking resistance and coatability in addition to the above performance.
It is a further object of the present invention to provide a flexible disk which comprises the above biaxially oriented polyester film of the present invention as a base film and has excellent electromagnetic conversion characteristics.
Other objects and advantages of the present invention will become apparent from the following description.
According to the present invention, firstly, the above objects and advantages of the present invention are attained by a biaxially oriented polyester film which has:
(A) a difference between refractive index on the front side and refractive index on the rear side of the film of 0 to 0.002;
(B) a heat shrinkage factor of 0 to 0.6% when it is heated at 105xc2x0 C. for 30 minutes under no load;
(C) a Young""s modulus of 4,609 MPa (470 kg/mm2) or more and a difference between the maximum value and the minimum value of Young""s modulus on the film plane of 981 MPa (100 kg/mm2) or less;
(D) a center line average roughness on the film plane of 2 to 10 nm; and
(E) a static friction coefficient between the films of 0.55 or less.
According to the present invention, secondly, the above objects and advantages of the present invention are attained by a laminate polyester film consisting of the above biaxially oriented polyester film of the present invention and an adhesive layer which contains colloidal particles having an average particle diameter of 0 to 200 nm and resin components including a water-soluble or water-dispersible copolyester having a sulfonate group as the main component and which is formed on both sides of the above film.
Further, according to the present invention, thirdly, the above objects and advantages of the present invention are attained by a flexible disk comprising the above laminate polyester film of the present invention and magnetic layers formed on the adhesive layers on both side of the laminate polyester film.
The polyester forming the biaxially oriented polyester film of the present invention is a polyester which comprises an aromatic dicarboxylic acid as the main dicarboxylic acid component and an aliphatic glycol as the main glycol component. The polyester is substantially linear and has film formability, particularly film formability by melt molding.
Illustrative examples of the aromatic dicarboxylic acid include terephthalic acid, naphthalenedicarboxylic acid, isophthalic acid, diphenoxyethanedicarboxylic acid, diphenyldicarboxylic acid, diphenylether dicarboxylic acid, diphenylsulfonedicarboxylic acid, diphenyl ketone dicarboxylic acid, anthracenedicarboxylic acid and the like.
Illustrative examples of the aliphatic glycol include polymethylene glycols having 2 to 10 carbon atoms such as ethylene glycol, trimethylene glycol, tetramethylene glycol, pentamethylene glycol, hexamethylene glycol and decamethylene glycol; and alicyclic diols such as cyclohexane dimethanol.
The polyester used for the production of the biaxially oriented polyester film of the present invention preferably comprises an alkylene terephthalate and/or an alkylene naphthalene dicarboxylate as the main recurring ingredient.
Out of the polyesters, polyethylene terephthalate- and polyethylene-2,6-naphthalene dicarboxylate-based polyesters which are a homopolymer and copolymer comprising terephthalic acid and/or 2,6-naphthalenedicarboxylic acid in an amount of 80 mol % or more based on the total of all the dicarboxylic acid components and ethylene glycol in an amount of 80 mol % or more based on the total of all the glycol components are particularly preferred.
In the case of a copolymer, less than 20 mol % of the total of all the acid components may consist of any one of the above dicarboxylic acids other than terephthalic acid and/or 2,6-naphthalenedicarboxylic acid, for example, an aliphatic dicarboxylic acid such as adipic acid or sebacic acid, or an alicyclic dicarboxylic acid such as cyclohexane-1,4-dicarboxylic acid.
Less than 20 mol % of the total of all the glycol components may consist of any one of the above glycols other than ethylene glycol, for example, an aromatic diol such as hydroquinone, resorcin or 2,2-bis(4-hydroxyphenyl)propane; an aliphatic diol having an aromatic ring such as 1,4-dihydroxydimethylbenzene; or a polyalkylene glycol(polyoxyalkylene glycol) such as polyethylene glycol, polypropylene glycol or polytetramethylene glycol.
The polyester in the present invention comprehends a polyester prepared by copolymerizing an oxycarboxylic acid such as an aromatic oxyacid exemplified by hydroxybenzoic acid or aliphatic oxyacid exemplified by w-hydroxycaproic acid in an amount of 20 mol % or less based on the total of the dicarboxylic acid component and the oxycarboxylic acid component. When the amount of the oxycarboxylic acid is more than 20 mol %, the obtained film can be hardly used as a base film due to deterioration in its physical and chemical properties.
The polyester in the present invention further comprehends a polyester prepared by copolymerizing a polycarboxylic acid component having a functionality of 3 or more or a polyhydroxy compound component such as trimellitic acid or pentaerythritol in such an amount that it is substantially linear, for example, 2 mol % or less based on the total of all the acid components. When the amount of the polycarboxylic acid component or polyhydroxy compound component is more than 2 mol %, the characteristic properties of a linear polymer are impaired and the film formability of the polymer lowers.
The above polyester is known per se and can be produced by a known melt polymerization method. Preferably, the obtained polyester has an intrinsic viscosity measured at 35xc2x0 C. in an o-chlorophenol solution of 0.4 to 0.9. When the intrinsic viscosity is lower than 0.4, the degree of polymerization is low, making it difficult to obtain desired physical properties. When the intrinsic viscosity is higher than 0.9, the degree of polymerization is high, making molding difficult.
The biaxially oriented polyester film of the present invention must have a difference between refractive index on the front side and refractive index on the rear side of the film of 0 to 0.002, preferably 0 to 0.001. When the difference is larger than 0.002, the obtained film is curled by curing after a magnetic layer is formed, thereby impairing the flatness of the obtained flexible disk. Thus, the flexible disk cannot be used.
The biaxially oriented polyester film must have a heat shrinkage factor of 0 to 0.6% when it is heated at 105xc2x0 C. for 30 minutes under no load. The heat shrinkage factor is preferably 0 to 0.5%, more preferably 0 to 0.2%. When the heat shrinkage is larger than 0.6%, the heat shrinkage factor of a flexible disk after a magnetic layer is formed becomes large, thereby causing such a problem as a track shift.
The biaxially oriented polyester film must have a Young""s modulus of 4,609 MPa (470 kg/mm2) or more, preferably 4,904 MPa (500 kg/mm2) or more, more preferably 5,590 MPa (570 kg/mM2) or more. When the Young""s modulus is smaller than 4,609 MPa (470 kg/mm2), the heat shrinkage factor of the obtained flexible disk becomes larger than that of the base film because large stress is applied to the film by tension which is applied when a coating for forming a magnetic layer is applied, or the like. Further, the Young""s modulus is preferably uniform on the film plane from the viewpoint of stiffness and the difference between the maximum value and the minimum value of Young""s modulus must be 981 MPa (100 kg/mm2) or less, preferably 686 MPa (70 kg/mm2) or less.
The biaxially oriented polyester film of the present invention must have a center line average roughness (Ra) on the both sides of the film of 2 to 10 nm, preferably 3 to 10 nm. When the center line average roughness is smaller than 2 nm, the film is easily scratched extensively during its production process and when the center line average roughness is larger than 10 nm, recording output lowers disadvantageously. The term xe2x80x9ccenter line average roughnessxe2x80x9d means a value obtained by measuring the surface of an adhesive layer when the adhesive layer is formed on the surface of the film.
Inert fine particles can be contained in the polyester forming the film to achieve the above center line average roughness. Preferably, first inert fine particles (A) having an average particle diameter of 0.01 to 0.5 xcexcm are contained in the film in an amount of 0.1 to 0.5 wt % and second inert fine particles (B) having an average particle diameter of 0.1 to 1.0 xcexcm are contained in the film in an amount of 0.001 to 0.1 wt %. The average particle diameter of the second inert fine particles (B) is larger than the average particle diameter of the first inert fine particles (A).
The average particle diameter of the first inert fine particles (A) is 0.01 to 0.5 xcexcm, preferably 0.01 to 0.3 xcexcm, more preferably 0.05 to 0.2 xcexcm. When the average particle diameter is smaller than 0.01 xcexcm, the slipperiness of the surface becomes unsatisfactory and when the average particle diameter is larger than 0.5 xcexcm, recording output lowers disadvantageously. The content of the first inert fine particles (A) is suitably selected from a range of 0.1 to 0.5 wt % to obtain a predetermined surface roughness.
The average particle diameter of the second inert fine particles (B) is 0.1 to 1.0 xcexcm, preferably 0.1 to 0.8 xcexcm, more preferably 0.2 to 0.8 xcexcm. When the average particle diameter is smaller than 0.1 xcexcm, the slipperiness of the surface becomes unsatisfactory and when the average particle diameter is larger than 1.0 xcexcm, recording output lowers disadvantageously. The content of the second inert fine particles (B) is suitably selected from a range of 0.001 to 0.1 wt % to achieve a predetermined friction coefficient. It is preferably 0.005 to 0.05 wt %.
The inert fine particles are preferably externally added particles. Internally precipitated particles are not preferred as main particles because it is difficult to control the particle size of the internally precipitated particles. Examples of the externally added particles include monodisperse particles and agglomerated particles such as calcium carbonate, colloidal silica, agglomerated silica, alumina and organic particles. Out of the monodisperse particles, colloidal silica and organic particles are preferred and spherical particles are more preferred as the main particles to achieve the reproducibility of surface roughness and reduce coarse protrusions.
The organic particles are preferably heat resistant polymer particles such as crosslinked divinylbenzene particles or crosslinked silicone particles.
The first inert fine particles and the second inert fine particles may be of the same chemical species or different chemical species.
The first inert fine particles are preferably at least one member selected from the group consisting of spherical silica particles and alumina particles. The second inert fine particles are preferably at least one member selected from the group consisting of heat resistant polymer particles and spherical silica particles.
When the first inert fine particles and the second inert fine particles are of the same chemical species, for example, spherical silica, it is desirable that the first inert fine particles and the second inert fine particles should differ from each other in the particle size peak value of a distribution curve.
When the first inert fine particles (A) and the second inert fine particles (B) are both spherical silica particles, the particle diameters of 99% or more of the spherical silica particles are preferably in the range of 0.5a to 2a (a is the average particle diameter of the spherical silica particles).
Further, the biaxially oriented polyester film of the present invention must have a static friction coefficient (xcexcs) between films of 0.55 or less. The dynamic friction coefficient (xcexcd) of the biaxially oriented polyester film is preferably 0.2 to 0.55. For example, when the handling properties of the film having a flat surface are poor in the film forming step or magnetic layer forming step, it is wrinkled or scratched while it is running over rolls or rolled in these steps.
Preferably, the biaxially oriented polyester film of the present invention has a curl value of 10 mm or less when measured after it is left at a temperature of 60xc2x0 C. and a humidity of 50% for 24 hours. The curl value is more preferably 6 mm or less, much more preferably 3 mm or less. When the curl value is larger than 10 mm, the film is easily curled at the time of curing after a magnetic layer is formed, whereby the flatness of the obtained flexible disk may be impaired, and the contact of the flexible disk with the head may become nonuniform or the distance between the floating head and the flexibe disk may become nonuniform, thereby reducing output.
The biaxially oriented polyester. film of the present invention has a thickness of preferably 20 to 80 xcexcm, more preferably 25 to 75 xcexcm. This thickness is especially preferred for a high-density magnetic recording flexible disk. Further, the film has the above surface roughness. These satisfy requirements for a high-density recording system. When the thickness of the film is smaller than 20 xcexcm, the rigidity of the film as a disk lowers and when the thickness is larger than 80 xcexcm, the flexibility of the film becomes unsatisfactory for recording and reproduction with the magnetic head.
The biaxially oriented polyester film of the present invention is preferably produced in accordance with a known sequential biaxial stretching method but may be produced by a simultaneous biaxial stretching method or with a simple testing apparatus.
For example, the biaxially oriented polyester film can be produced by melt extruding a sufficiently dried polyester resin at a temperature of melting point to (melting point+70)xc2x0 C., quenching the resin on a casting drum to produce an unstretched film, biaxially stretching the unstretched film sequentially or simultaneously and heat setting it.
Biaxial stretching is preferably sequential biaxial stretching. The unstretched film is stretched to 2.5 to 5.0 times in a longitudinal direction at a temperature of (Tgxe2x88x9210) to (Tg+70)xc2x0 C. (Tg: secondary transition point of the polyester). Preferably, the stretching temperature is adjusted or the film is preheated before stretching at this point to ensure that the difference between the front surface temperature and the rear surface temperature of the film at the time of stretching should become 10xc2x0 C. or less so that the difference of refractive index on the front side and refractive index on the rear side of the film becomes 0.002 or less.
Thereafter, the film is stretched to 2.5 to 5.0 times in a direction perpendicular to the above direction (transverse direction when the film was first stretched in a longitudinal direction) at a temperature of Tg(xc2x0 C.) to (Tg+70)xc2x0 C. In this case, the area draw ratio is preferably 9 to 22 times, more preferably 12 to 22 times.
The biaxially oriented film may be heat set at a temperature of (Tg+70)xc2x0 C. to Tm(xc2x0 C.). For example, a polyethylene terephthalate film is preferably heat set at 190 to 230xc2x0 C. The heat setting time is 1 to 60 sec., for example. The stretching conditions in longitudinal and transverse directions are selected to ensure that the physical properties of the biaxially oriented polyester film in both directions should become almost the same, the Young""s modulus in a plane direction should become 4,609 MPa (470 kg/mm2) or more, and the difference between the maximum value and the minimum value of Young""s modulus should become 981 MPa (100 kg/mm2) or less. In the case of simultaneous biaxial stretching, the above stretching temperature, draw ratio, heat setting temperature and the like can be adopted.
Three-stage stretching or four-stage stretching may also be adopted that the biaxially oriented polyester film is further stretched in a longitudinal direction and/or transverse direction as required.
A description is subsequently given of a laminate polyester film provided by the present invention. This laminate polyester film comprises the above biaxially oriented polyester film of the present invention and an adhesive layer formed on both sides of the film. The adhesive layer contains colloidal particles having an average particle diameter of 10 to 200 nm and resin components mainly including a water-soluble or water-dispersible copolyester having a sulfonate salt group. Although this adhesive layer is aimed to improve adhesion to a magnetic layer formed thereon, it has slipperiness because it has colloidal particles on the surface.
The sulfonate salt group contained in the molecule of the above water-soluble or water-dispersible copolyester is preferably a group represented by the formula xe2x80x94SO3M (M is a metal atom, ammonium group or quaternary amine having the same equivalent to that of xe2x80x94SO3). The amount of the sulfonate salt group is preferably 5 to 18 mol %, more preferably 8 to 18 mol %, particularly preferably 9 to 12 mol % based on the total of all the dicarboxylic acid components. When the amount is smaller than 5 mol %, the water dispersibility and coatability of the copolyester are apt to deteriorate and when the amount is larger than 18 mol %, adhesion and blocking resistance are liable to lower.
To introduce the sulfonate salt group into the molecule of the polymer, a bifunctional compound having a sulfonate salt group, such as a dicarboxylic acid component having a sulfonate salt group exemplified by 5-Na sulfoisophthalic acid, 5-ammonium sulfoisophthalic acid, 4-Na sulfoisophthalic acid, 4-methylammonium isophthalic acid, 2-Na sulfoisophthalic acid, 5-K sulfoisophthalic acid, 4-K sulfoisophthalic acid or 2-K sulfoterephthalic acid, or a dihydroxy compound having a sulfonate salt group represented by the following general formula (1) or (2) is preferably used. 
(m and n are an integer of 1 to 2, with the proviso that m+n is 2 to 4.) 
(p and q are an integer of 1 to 2, with the proviso that p+q is 2 to 4.)
Out of these, dicarboxylic acid components having a sulfonate salt group are preferred and may be used in combination of two or more.
Illustrative examples of the acid component forming the above water-soluble or water-dispersible copolyester include terephthalic acid, isophthalic acid, phthalic acid, 2,6-naphthalenedicarboxylic acid, 4,4xe2x80x2-diphenyldicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, adipic acid, sebacic acid, trimellitic acid, pyromellitic acid, dimeric acid and the like. They may be used in combination of two or more.
Further, an unsaturated polybasic acid such as maleic acid, fumaric acid or itaconic acid, or hydroxycarboxylic acid such as p-hydroxybenzoic acid or p-(xcex2-hydroxyethoxy)benzoic acid may be used in a small amount. The amount of the unsaturated polybasic acid component or hydroxycarboxylic acid component is preferably 10 mol % or less, more preferably 5 mol % or less. When the amount is larger than 10 mol %, the chipping resistance and blocking resistance of the polyester resin lower.
Illustrative examples of the dihydroxy compound component forming the above water-soluble or water-dispersible copolyester include ethylene glycol, 1,4-butanediol, neopentyl glycol, diethylene glycol, triethylene glycol, propylene glycol, 1,6-hexanediol, 1,4-cyclohexane dimethanol, xylylene glycol, dimethylolpropionic acid, glycerin, trimethylol propane, poly(ethylene oxide)glycol, poly(tetramethylene oxide)glycol, bisphenol A adduct with alkylene oxide and the like. They may be used in combination of two or more.
The secondary transition point (DSC method) of the above water-soluble or water-dispersible copolyester is preferably 50 to 85xc2x0 C., more preferably 60 to 80xc2x0 C., much more preferably 65 to 75xc2x0 C. When the secondary transition point is lower than 50xc2x0 C., chipping resistance and blocking resistance are apt to lower and when the secondary transition point is higher than 850xc2x0 C., adhesion or coatability are liable to lower.
The above water-soluble or water-dispersible copolyester can be produced by any methods known as polyester production methods. The types and amounts of the dicarboxylic acid component and the dihydroxy compound component can be suitably selected in consideration of the above-described characteristic properties of the polymer. The number average molecular weight of the copolyester can be freely selected but it is preferably 5,000 to 28,000. When the number average molecular weight is smaller than 5,000, the desirable characteristic properties of the polymer are hardly obtained, and chipping resistance and blocking resistance are apt to lower. When the number average molecular weight is larger than 28,000, it is difficult to form a uniform coating solution, and adhesion and coatability are liable to lower.
The adhesive layer may further contain a surfactant composed of a polyoxyethylene alkylphenyl ether having an HLB value of 11 to 20 as a resin component other than the above water-soluble or water-dispersible copolyester.
The above polyoxyethylene alkylphenyl ether surfactant is a polyoxyethylene nonylphenyl ether represented by the following general formula (3). 
This compound has an HLB value of 12.6 when n is 8.5 and 17.1 when n is 30.
When the HLB value of the polyoxyethylene alkylphenyl ether surfactant is larger than 20, hydrophilic nature become too strong and the coatability of a coating solution is liable to lower and when the HLB value is smaller than 11, the characteristic properties of the interface become weak and the stability and coatability of the coating solution are apt to lower.
The polyoxyethylene alkylphenyl ether surfactant is preferably contained in an amount of 1 to 100 parts by weight based on 100 parts by weight of the water-soluble or water-dispersible copolyester.
When the amount of the above surfactant is smaller than 1 part by weight based on 100 parts by weight of the water-soluble or water-dispersible copolyester, the characteristic properties of the interface become slightly weak and the stability of the coating solution is apt to lower and when the amount is larger than 100 parts by weight, the content of the copolyester in the coating solution lowers with the result that adhesion tends to lower.
Another polyoxyethylene alkylphenyl ether having a different HLB value of 11 to 20 may be used in combination with the above polyoxyethylene alkylphenyl ether. The combination of polyoxyethylene alkylphenyl ethers is preferably a combination of a polyoxyethylene alkylphenyl ether having an HLB value of 11 to 13 and a polyoxyethylene alkylphenyl ether having an HLB value of more than 13 and 20 or less.
Particularly preferably, the above combination consists of 0.5 to 50 parts by weight of the polyoxyethylene alkylphenyl ether surfactant having an HLB value of 11 to 13 and 0.5 to 99.5 parts by weight of the polyoxyethylene alkylphenyl ether surfactant having an HLB value of 13 to 20 based on 100 parts by weight of the water-soluble and water-dispersible copolyester resin. The HLB value of the surfactant having an HLB value of 13 to 20 is preferably 13 to 19, more preferably 17 to 18.
Since the hydrophilic nature and lipophilic nature of the surfactant used are well balanced by this combination, a copolyester having excellent coatability as well as blocking resistance and adhesion can be obtained.
Further, the adhesive layer contains colloidal particles having an average particle diameter of 10 to 200 nm.
The average particle diameter of the colloidal particles is preferably 30 to 150 nm, more preferably 50 to 120 nm. When the average particle diameter is smaller than 10 nm, the particles are too small, thereby making it difficult to exhibit the effect of improving blocking resistance and the chipping resistance of the magnetic layer and when the average particle diameter is larger than 200 nm, the particles readily fall off and the chipping resistance of the base film is liable to deteriorate.
Preferably, the colloidal particles have a volume shape coefficient (f) represented by the following equation (4) of 0.4 to xcfx80/6:
f=V/D3xe2x80x83xe2x80x83(4)
wherein f is a volume shape coefficient, V is the average volume of particles (m3), and D is the average maximum diameter (m) of the particles.
When the volume shape coefficient (f) is smaller than 0.4, the effect of improving blocking resistance and the chipping resistance of the magnetic layer is hardly exhibited in full.
The amount of the colloidal particles is preferably 5 to 150 parts by weight, more preferably 10 to 100 parts by weight, much more preferably 20 to 80 parts by weight based on 100 parts by weight of the above water-soluble or water-dispersible copolyester. When the amount is smaller than 5 parts by weight, the effect of improving blocking resistance is not exhibited in full and when the amount is larger than 150 parts by weight, the particles are liable to coagulate and the chipping resistance of the base film is apt to deteriorate.
Preferably, the thickness (nm) of the adhesive layer has the relationship represented by the following expression (5) with the average particle diameter (nm) of the colloidal particles.
0.2xe2x89xa6(thickness of adhesive layer/average particle diameter of colloidal particles)xe2x89xa63.0xe2x80x83xe2x80x83(5)
The relationship is more preferably represented by the following expression (6), particularly preferably the following expression (7).
0.4 xe2x89xa6(thickness of adhesive layer/average particle diameter of colloidal particles)xe2x89xa62.0xe2x80x83xe2x80x83(6)
0.5xe2x89xa6(thickness of adhesive layer/average particle diameter of colloidal particles)xe2x89xa61.5xe2x80x83xe2x80x83(7)
When the value of (thickness of adhesive layer/average particle diameter of colloidal particles) is smaller than 0.2, the particles are readily chipped off and the chipping resistance of the base film is liable to deteriorate and when the value is larger than 3.0, the effect of improving blocking resistance and the chipping resistance of the magnetic layer is hardly exhibited in full.
The colloidal particles are preferably spherical silica particles or heat resistant polymer particles.
The spherical silica particles are as described above but the heat resistant polymer particles are preferably particles of a polymer having a 5% heat weight reduction temperature under a nitrogen gas atmosphere of 310xc2x0 C. or more, more preferably 330xc2x0 C. or more, particularly preferably 350xc2x0 C. or more. Illustrative examples of the particles include crosslinked silicone resin particles, crosslinked acrylic resin particles, crosslinked polystyrene particles, Teflon particles, polyimide particles and the like. Out of these, crosslinked silicone resin particles and crosslinked acrylic resin particles are preferred and core-shell type particles are particularly preferred from the viewpoint of adhesion to a binder.
Preferably, the adhesive layer comprises 100 parts by weight of a water-soluble or water-dispersible copolyester, 1 to 100 parts by weight of a polyoxyethylene alkylphenyl ether having an HLB value of 11 to 20 and 5 to 150 parts by weight of colloidal particles having an average particle diameter of 10 to 200 nm.
A coating solution for forming the adhesive layer, preferably an aqueous solution may contain as required other resin, antistatic agent, lubricant, filler, colorant, ultraviolet light absorber, antioxidant, surfactant, crosslinking agent such as melamine, epoxy or aziridine compound for improving the heat resistance and blocking resistance of an adhesive. The aqueous coating solution may contain a small amount of an organic solvent.
The solids content of the coating solution may be determined optionally but is preferably 1 to 15 wt %, more preferably 1 to 12 wt %, more preferably 1 to 10 wt %.
The amount of wet coating is preferably 0.5 to 20 g, more preferably 1 to 10 g per 1 m2 of the running film (uniaxially stretched film) and the thickness of a dried coating film is preferably 5 to 200 nm, more preferably 10 to 100 nm.
Any known coating methods may be employed. For example, roll coating, gravure coating, reverse coating, roll brushing, spray coating, air knife coating, impregnation and curtain coating may be used alone or in combination.
The coating solution may be applied to the biaxially oriented polyester film but is preferably applied to a longitudinally stretched polyester film.
The uniaxially stretched polyester film to which an aqueous coating solution has been applied is dried, stretched in a transverse direction, stretched again in a longitudinal direction as required and introduced to the following heat setting step. For example, the longitudinally stretched polyester film to which the aqueous coating solution has been applied is introduced to a stenter to be stretched in a transverse direction, stretched in a longitudinal direction again as required and heat set. During this, the coating solution is dried to form a continuous coating film on the film. Drying may be carried out before or after stretching in a transverse direction.
The biaxially oriented polyester film and the laminate polyester film having an adhesive layer on both sides of the present invention are advantageously used as a base film for a flexible disk.
According to the present invention, there is provided a flexible disk comprising the laminate polyester film of the present invention and a magnetic layer formed on the adhesive layer on both sides of the laminate polyester film.
Technologies known per se may be used for the magnetic layer and forming the same may be used.
The following examples are provided for the purpose of further illustrating the present invention but are in no way to be taken as limiting. xe2x80x9cPartsxe2x80x9d and xe2x80x9c%xe2x80x9d in the following examples or comparative examples are based on weight unless otherwise stated. The physical property values and characteristic properties in the present invention were measured in accordance with the following methods.
(1) difference between refractive index on front side and refractive index on rear side of film.
The refractive index of the film is measured using sodium D-rays (589 nm) as a light source, Abbe refractometer and methylene iodide as an intermediate solution. The refractive indices on the front and rear sides of the film are obtained by sampling 18 films at a pitch of 10xc2x0 from a certain direction on the film plane, for example, a longitudinal direction and measuring refractive indices on the front and rear sides of each sample in the same direction. When the difference between refractive index on the front side and refractive index on the rear side of the film measured as described above is in the range of 0 to 0.002 in all of the 18 samples, all the samples satisfy the difference of refractive index in the present invention. The largest difference of refractive index between the front and rear sides is the largest out of the values obtained by the above measurement.
(2) heat shrinkage factor
An about 30 cm (width)xc3x9730 cm (length) film whose size has been measured accurately is suspended in an oven heated at 105xc2x0 C., retained under no load for 30 minutes, taken out from the oven and returned to room temperature to measure its size.
The heat shrinkage factor is defined by the following equation.
heat shrinkage factor=(xcex94L/L0)xc3x97100 xcex94L=|L0xe2x88x92L|
(L0 is the length of the film before a heat treatment and L is the length in the same direction of the film after the heat treatment)
The heat shrinkage factor of the present invention is satisfied when a value obtained by sampling 6 films at a pitch of 30xc2x0 from every direction on the film plane, for example, the longitudinal direction of the film and measuring each sample as described above is in the range of 0 to 0.6%.
(3) Young""s modulus
The Young""s modulus of the film is measured by setting a film sample having a width of 10 mm and a chuck interval of 100 mm in a tensile tester and pulling it at 23xc2x0 C., 65%RH and a pulling rate of 200 mm/min.
The Young""s modulus of the present invention is satisfied when a value obtained by sampling 18 films at a pitch of 10xc2x0 C. from every direction on the film plane, for example, the longitudinal direction of the film and measuring each sample as described above is 4,609 MPa (470 kg/mm2) or more. When the difference between the maximum value and the minimum value of Young""s modulus measured for each sample is 981 MPa (100 kg/mm2) or less, the difference of Young""s modulus in the present invention is satisfied.
(4) film surface roughness (center line average surface roughness Ra)
The surface roughness of the film is measured with the tracer type surface roughness meter of Kosaka Kenkyusho Co., Ltd. at a tracer load of 80 mg, a measurement length of 4 mm and a cut-off of 0.25 mm. The definition of Ra is given in xe2x80x9cMeasurement And Evaluation Methods Of Surface Roughnessxe2x80x9d written by Jiro Nara (published by Sogo Gijutsu Center in 1983), for example.
(5) static friction coefficient (xcexcs), dynamic friction coefficient (xcexcd)
A load of 200 g (W: g) is placed on two cut films (samples) measuring 75 mm (width)xc3x97100 mm (length), the upper film is caused to travel at a speed of 150 mm/min, and the static friction coefficient (xcexcs) is calculated from tensile force (Fs: g) when the film begins to travel and the dynamic friction coefficient (xcexcd) is calculated from force (Fd: g) when the film is traveling based on the following equations.
xcexcs=Fs(g)/W(g)
xcexcd=Fd(g)/W(g)
The film sample has been left at 23xc2x0 C. and 65%RH for 24 hours.
(6) total thickness of film
10 films are placed one upon another so that dust is not contained therein and the thickness of the 10 films is measured with an intermittent electronic micrometer to calculate the thickness of each film.
(7) curl value
A 8.9 mm-diameter (3.5 inches) circular sample is left at a temperature of 60xc2x0 C. and a humidity of 50% for 24 hours, taken out and left at room temperature (humidity of 50%) for 1 hour. The gap between the edge portion of the film and a vertical axis when the sample is suspended perpendicularly is measured.
This measurement is carried out at a pitch of 10xc2x0 in a circumferential direction and the maximum value is taken as a curl value (unit was mm) (measured clockwise with a longitudinal direction taken as 0xc2x0).
(8) handling properties
The wrinkling of the film caused by poor slipperiness over pass rolls is judged based on the following criteria.
◯: no practical problem
X: film is wrinkled, causing practical problem
(9) secondary transition point
The secondary transition point of a polyester is obtained by measuring with DSC at a temperature elevation rate of 20xc2x0 C./min.
(10) average particle diameter of particles
This is measured using the CP-50 Centrifugal Particle Size Analyzer (trade name) of Shimadzu Corporation. A particle diameter equivalent to 50 mass percent is read from the cumulative curve of the particles of each diameter and the amount thereof calculated based on the obtained centrifugal sedimentation curve and taken as the above average particle diameter (refer to xe2x80x9cParticle Size Measurement Technologyxe2x80x9d issued by Nikkan Kogyo Press, pp. 242-247, 1975).
(11) adhesion (evaluation of MAG scratches)
The following xe2x80x9cevaluation magnetic coatingxe2x80x9d is applied to the coating surface of a sample film with a Meyer""s bar to ensure that the thickness of the coating film after drying should become about 4 xcexcm and dried at 100xc2x0 C. for 3 minutes. Thereafter, the coating film is aged at 60xc2x0 C. for 24 hours. xe2x80x9cevaluation magnetic coatingxe2x80x9d (1) 30 parts of a polyester resin (Vilon 20SS (trade name) of Toyobo Co., Ltd.), (2) 10 parts of a vinyl chloride-vinyl acetate resin (Eslec A (trade name) of Sekisui Chemical Co., Ltd.), (3) 3 parts of a dispersant (Lecion P (trade name) of Riken Vitamin Co., Ltd.) and (4) 260 parts of a magnetizing agent (CTX-970 (trade name) of Toda Kogyo Co., Ltd.) in terms of solid matter are dissolved in a mixed solvent of methyl ethyl ketone, toluene and methyl isobutyl ketone to prepare a 30% solution and dispersed with a sand grinder for 2 hours. Thereafter, 12 parts of a crosslinking agent (Colonate L (trade name) of Nippon Polyurethane Co., Ltd.) in terms of solid matter is added to the dispersion and stirred well to obtain a magnetic coating.
A surface coated with the thus obtained magnetic coating is scratched with a scratch tester covered with fibers having high rigidity (registered trademark xe2x80x9cKevlarxe2x80x9d yarn) under a load of 1 kg to evaluate scratches on the surface coated with the magnetic coating. This evaluation method corresponds well with scratches (MAG scratches) on the magnetic surface after the formation of a magnetic layer in the production of a magnetic medium.
 less than Criteria greater than 
⊚: MAG does not peel off at all
◯: MAG peels off slightly
X: MAG peels off significantly
(12) coatability
The adhesive layer formed surface of the film is observed with the eye to evaluate coating nonuniformity.
 less than Criteria greater than 
⊚: almost no coating nonuniformity is seen
◯: slight coating nonuniformity is seen
X: coating nonuniformity is large
(13) blocking resistance
After two polyester films having an adhesive layer on both sides are placed one upon the other and left at a temperature of 60xc2x0 C. and a humidity of 80%RH under a load of 14.7 MPa (150 kg/cm2) for 65 hours, the peel strength of the two films is measured with a tensile tester and evaluated.
 less than Criteria greater than 
⊚: peel strength is 0 to 10 g/10 cm
◯: peel strength is 11 to 20 g/10 cm
xcex94: peel strength is 21 to 30 g/10 cm
X: peel strength is 31 g/10 cm to breakage